Carbon nanofiber materials and lubricants

ABSTRACT

Nickel-containing carbon nanofiber (NiCNF) materials and lubricants comprising the nanofiber materials are described herein. The NiCNF materials comprise a plurality of elongate nickel-containing carbon nanofibers. Each NiCNF comprises a carbon nanofiber shaft that is capped at one end by a nickel nanoparticle encapsulated in multiple layers of graphitic carbon. The nanofibers typically have a generally circular cross-section (i.e., in the direction perpendicular to the length-axis of the fibers). The NiCNF materials as useful, e.g., in lubricant compositions, which also are described herein.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.

FIELD OF THE INVENTION

This invention relates to carbon nanofiber materials and lubricant compositions comprising the carbon nanofiber materials. More particularly, this invention relates to nanofiber materials comprising a carbon and nickel nanoparticle, and lubricant compositions comprising the nanofiber materials.

BACKGROUND OF THE INVENTION

The energy efficiency, durability, and environmental compatibility of all kinds of moving mechanical systems (including engines) are closely related to the effectiveness of the lubricants being used on their rolling, rotating, and sliding surfaces. Therefore, lubricants play a vital role in machine life, efficiency, and overall performance. Poor or inefficient lubrication always result in higher friction and severe wear losses, which can in turn adversely impact the performance and durability of mechanical systems. In particular, progressive wear due to inadequate lubrication is one of the most serious causes of component failure. Inadequate lubrication can also cause significant energy losses in the above-mentioned industrial systems mainly because of high friction.

Currently, there are numerous solid lubricants available at sizes ranging from 1 nm to more than 500 nm in powder forms. The finer solids (i.e. 1 to 30 nm range) are mostly made of nanostructured carbon materials such as C₆₀, nano-tubes, nano-fibers, and nano-onions, while intermediate range lubricants (30 to 100 nm) typically include inorganic solids such as MoS₂, WS₂, hexagonal crystalline boron nitride (h-BN), and pure metals (e.g., gold, silver, tin, bismuth, etc.). WS₂ is synthesized typically in the form of fullerene-like particles and hence it is often referred to as inorganic fullerene or, IF. Many of these materials are manufactured using a bottom-up approach involving multi-step chemical synthesis routes (e.g. gas phase chemical processing, combustion synthesis, sonochemistry, etc.) and the uses of environmentally unsafe chemicals. Many current processes also generate large amounts of toxic by-products to deal with after the manufacturing.

Carbon particles, fibers, and nanotubes are utilized in a number of industrially significant applications in addition to lubricants, such as catalysts, electrode materials for alkali metal batteries, and chemical sorbants. In many cases, carbon nanomaterials (e.g., particles, tubes or fibers) are particularly useful in such applications. Carbon nanomaterials have been produced by a number of processes. The chemical and physical properties of carbon nanomaterials can be highly dependent on the physical form of the materials (e.g., physical dimensions, shape, presence or absence of cavities or pores (tubes, hollow particles, etc.), as well as the chemical nature (e.g., metals, metalloids, oxides) and the physical form (e.g., crystalline, amorphous) of any encapsulated or associated materials. In many cases, the form of the carbon materials and any associated, encapsulated or incorporated non-carbon materials (metals, metalloids, metal oxides, and the like) is highly dependent on the conditions (e.g., starting materials, temperature, pressure, etc.) used to produce the carbon nanomaterials.

Conventional lubricants comprising carbon do not yet meet the expectations of providing the performance requirements of motorized and mechanical devices. Carbon is an extremely versatile material that exists in numerous forms with diverse physical, chemical, electrical and electrochemical properties. Certain carbonaceous particles, such as carbon nano-onions, carbon nanofibers, carbon nanotubes and submicron graphite particles have all been considered for lubrication purposes in the past, but generally have been found to be expensive, ineffective, and/or difficult to scale-up.

There is an ongoing need for new carbon materials, particularly nanomaterials, as well as lubricant compositions comprising new carbon nanomaterials, which are, e.g., environmentally friendly or benign, and which can provide reduced friction and wear. The present invention addresses these needs.

SUMMARY

Nickel-containing carbon nanofiber (NiCNF) materials and lubricants comprising the nanofiber materials are described herein. The NiCNF materials comprise a plurality of elongate nickel-containing carbon nanofibers. Each NiCNF comprises a carbon nanofiber shaft that is capped at one end by a nickel nanoparticle encapsulated in multiple layers of graphitic carbon. The nanofibers typically have a generally circular cross-section (i.e., in the direction perpendicular to the length-axis of the fibers). The individual nanofibers typically have a length in the range of about 100 nanometers (nm) to about 2 micrometers (μm) and a fiber diameter (i.e., the diameter of the generally circular cross-section) in the range of about 30 nm to about 100 nm. The nickel nanoparticles encapsulated by the carbon nanofibers typically have a diameter in the range of about 10 nm to about 30 nm. The nanofibers are solid fibers, i.e., they are not hollow.

In some preferred embodiments, the nanofibers of the NiCNF material have a fiber diameter in the range of about of about 30 to about 100 nm, and a length in the range of about 100 nm to about 2 μm; the nickel nanoparticle has a diameter in the range of about 10 to about 30 nm; and the graphitic carbon encapsulating the nickel nanoparticle has a thickness in the range of about 4 to about 10 nm.

The NiCNF materials optionally can include one or more other form of non-NiCNF carbon, such as carbon nanotubes, carbon fibers without encapsulated nickel, and non-fibrous particulate carbon (e.g., generally spheroidal particles, oblate spheroidal particles, irregularly shaped particles, and the like). Preferably, the NiCNF material comprises less than 60 percent by weight (wt %) of non-NiCNF carbon, based on the total dry weight of carbon present in the material, e.g., less than 50, 40, 30, 25, 20 or 10 wt % thereof. Optionally, the NiCNF material can include one or more other form of nickel (i.e., non-encapsulated nickel, or nickel encapsulated in another form of carbon). Preferably, the NiCNF material includes less than about 30 w % of such other forms of nickel, based on the total weight of nickel in the material, e.g., less than about 25, 20, 15, 10, or 5 wt % thereof. Such other forms of carbon and/or nickel can be present, e.g., as impurities formed during the preparation of the nickel-containing carbon nanofibers, or optionally can be intentionally added components. The NiCNF material preferably contains not more than about 50 wt % of carbon that is not part of the nickel-containing carbon nanofibers (i.e., non-NiCNF carbon). Preferably, at least about 50% of the nickel in the NiCNF material is encapsulated at an end of a carbon nanofiber shaft.

In another aspect, lubricant compositions are described, which comprise, consist essentially of, or consist of the NiCNF materials optionally suspended in an aqueous medium or non-aqueous liquid medium (e.g., a natural or synthetic hydrocarbon-based material such as an oil, a wax, or a grease). The NiCNF material can be present in a liquid lubricant composition at a concentration, e.g., in the range of about 0.01 to about 15 wt %, preferably about 0.05 to about 5 wt %, about 0.1 to about 2 wt %, or 0.5 to about 1 wt %. In one lubricant composition embodiment, the individual nanofibers of the NiCNF material have a fiber diameter in the range of about of about 30 to about 100 nm, a length in the range of about 100 nm to about 2 μm, the nickel nanoparticle has a diameter in the range of about 10 to about 30 nm, and the graphitic carbon encapsulating the nickel nanoparticle has a thickness in the range of about 4 to about 10 nm.

In one embodiment, the present invention provides a lubricant composition comprising a NiCNF material as described herein suspended in a liquid hydrocarbon carrier, preferably at a NiCNF concentration in the range of about 0.1 to about 1 wt %. Optionally, the composition can further a surfactant (e.g., to aid in suspension of the NiCNF material).

Optionally, the liquid hydrocarbon carrier comprises a poly alpha olefin. Preferred poly alpha olefin (PAO) materials have a kinematic viscosity in the range of about 2 to about 10 centistokes (cSt) at about 100° C., e.g., a PAO with a viscosity of about 2 cSt or a PAO with a viscosity of about 4 cSt.

The lubricant composition also can optionally comprise a surfactant (e.g., a non-ionic surfactant, such as sorbitan trioleate. The surfactant, if utilized, can be present in the composition at a concentration in the range of about 1000 to about 20000 parts-per-million (ppm).

In another aspect, a method of preparing a NiCNF material is provided. The method comprises heating nickel(II) acetate neat in a sealed pressure reactor at a temperature of about 650 to about 800° C. at an autogenically produced pressure of about 300 to about 600 pounds-per-square inch (psi) under an inert atmosphere (e.g., nitrogen or argon).

The NiCNF materials described herein are particularly useful as additive materials in lubricant compositions for use in metal lubrication, where the nickel from the nanofiber materials unexpectedly has been found to form a nickel film in areas of wear on the metal surface. The nickel film fills in worn areas and negates or ameliorates wear that can occur with moving metal parts even in the presence of a lubricant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides (a) a scanning electron micrograph (SEM) of an end portion of a nickel-tipped carbon nanofiber (NiCNF) 10 comprising carbon nanofiber shaft 12 and encapsulated nickel nanoparticle 14; and (b) a high resolution SEM image showing a portion of nickel nanoparticle 14 encapsulated within layers of graphitic carbon 16.

FIG. 2 illustrates wear scars observed in evaluation of a NiCNF-containing lubricant at 20° C. (Panel (a)) and 100° C. (Panel (c)); Panel (b) shows a magnification of the wear scar of Panel (a); Panel (d) shows a magnification of the wear scar of Panel (c).

FIG. 3 illustrates wear scars observed in evaluation of a NiCNF-containing lubricant (Panel (a)) and comparative lubricant without any NiCNF present (Panel (b)).

DETAILED DESCRIPTION

New nickel-containing carbon nanofiber materials are described comprising a plurality of elongate nickel-containing carbon nanofibers. An individual NiCNF of the plurality of nanofibers comprises a carbon nanofiber shaft capped at one end with a nickel nanoparticle. The nickel nanoparticle is encapsulated by multiple layers of graphitic carbon at the end of the carbon nanofiber shaft.

Typically, the nickel nanoparticle, which can have a diameter in the range of about 10 to about 30 nm (e.g., an average diameter of about 15 to about 25 nm), is encapsulated by multiple layers of graphitic carbon having a total thickness of about 4 to 10 nm (e.g., an average of about 7 to 8 nm).

The nanofibers can be of any length, although generally the nanofibers have a length of about 100 nm to 2 μm (e.g., about 300 nm to about 1 μm), with an average length in the range of about 200 nm to about 800 nm.

The nanofibers have a generally circular transverse cross-section (i.e., perpendicular to the longest dimension (length) of the fiber) with fiber diameters in the range of about 25 to about 100 nm, and an average diameter in the range of about 40 to about 70 nm. The diameter of the shaft can be relatively uniform or can vary along the length of the shaft (i.e., a non-uniform shaft diameter).

Generally, at least about 40 percent (typically at least about 40 to about 60%; e.g., 50%) of carbon nanofibers within the NiCNF material include the encapsulated nickel nanoparticle. As used herein, the term “nanofiber” refers to a fiber having a cross-sectional diameter of less than 100 nm. Additionally, nanofibers can be generally straight or can be curved (e.g., coiled).

The carbon nanofiber shafts of the NiCNF materials are solid, i.e., not hollow, which distinguishes these materials from carbon nanotubes, which are hollow. The NiCNF materials optionally can include of other forms of carbon besides the nickel-containing carbon nanofibers. Preferably, the NiCNF materials contain less than about 60 percent by weight (wt %), preferably less than 50, 40, 30, 25, 20 or 10 wt % of non-NiCNF carbon. Non-limiting examples of other forms of carbon that may be present in the NiCNF materials include carbon nanotubes, carbon fibers without encapsulated nickel, and non-fibrous particulate carbon (e.g., generally spheroidal particles, oblate spheroidal particles, irregularly shaped particles, and the like). In addition or alternatively, the NiCNF materials can include nickel that is not encapsulated at an end of a carbon nanofiber, e.g., non-encapsulated nickel, or nickel that is encapsulated in another form of carbon such as a carbon nanotube, a non-fibrous carbon nanoparticle, or a non-fibrous carbon microparticle. Preferably, the NiCNF materials include less than about 25, 20, 15, 10, or 5 wt % of nickel that is not encapsulated on the end of a carbon nanofiber. The other forms of carbon and/or nickel can be present, e.g., as impurities formed during the preparation of the NiCNF materials, or optionally can be intentionally added components.

The NiCNF materials described herein can be prepared by thermal decomposition of nickel(II) acetate under autogenic (i.e., self-generated) pressure. Typically, the nickel acetate is heated under an inert atmosphere (e.g., nitrogen or argon) at a temperature in the range of about 650 to about 800° C. (preferably about 700° C.) in a sealed reactor sized such that an internal pressure in the range of about 300 to about 600 psi is attained upon heating, e.g., due to gases generated during the decomposition of the nickel acetate to form the NiCNF material and any headspace or interstitial gas than may be present in the sealed reactor. The resulting NiCNF products comprise solid carbon nanofibers (i.e., not hollow) in which individual fibers comprise a carbon nanofiber shaft having a nickel nanocrystal encapsulated within multiple layers of graphitic carbon at an end of the shaft. As noted above, minor amounts of other forms of carbon and/or nickel can be present as by-products.

In another aspect, lubricant compositions are described, which comprise, consist essentially of, or consist of a NiCNF material, optionally suspended in a liquid medium. In certain embodiments, the liquid medium can be an aqueous liquid or a non-aqueous liquid medium. In some embodiments, the NiCNF material is suspended in a non-aqueous liquid such as a natural or synthetic hydrocarbon-based oil, a poly(alkylene glycol) such as poly(ethylene glycol), a wax, or a grease. Non-limiting examples of suitable liquid media include materials comprising, consisting essentially of, or consisting of water, an alcohol, a glycol, glycerol, a poly(alkylene glycol) (e.g., poly(ethylene glycol), poly(propylene glycol), poly(ethylene glycol-co-propylene glycol), and the like), a hydrocarbon, other lubricious organic solvents or polymers (e.g., glycol ethers, glycerol ethers, glycerol esters, glycol esters, poly(alkylene glycol) esters, and poly(alkylene glycol) ethers), a PAO, or a combination of two or more of the foregoing liquids. In some preferred embodiments, the liquid carrier comprises a PAO. Preferred PAO materials have a kinematic viscosity in the range of about 2 to about 10 centistokes (cSt) at about 100° C. In some preferred embodiments the liquid carrier comprises a PAO having a kinematic viscosity of about 2 cSt or about 4 cSt. The NiCNF material can be present in a liquid lubricant composition at a concentration, e.g., in the range of about 0.01 to about 15 wt %, preferably about 0.05 to about 5 wt %, about 0.1 to about 2 wt %, or 0.5 to about 1 wt %.

Optionally, the lubricant composition can further comprise a surfactant such as a nonionic surfactant (e.g., sorbitan trioleate), for example, to aid in suspension of the nanofiber material. Non-limiting examples of other nonionic surfactants include sorbitan sesquioleate (SSO) and SURFONIC LF-17 (an ethoxylated and propoxylated linear C₁₂-C₁₂ alcohol). Optionally, the surfactant, if utilized, is present in the composition at a concentration in the range of about 1000 to about 20000 parts-per-million (ppm), more preferably about 5000 to about 10000 ppm.

The following examples are provided to illustrate certain preferred embodiments of the present invention, and are not to be considered a limiting the scope of the appended claims. The examples demonstrate, in particular, the versatility of autogenic reactions in synthesizing carbon-based materials with a diverse range of particle morphologies, conducive to their use as additives for lubrication technology.

Autogenic Pressure Reactor.

A typical reactor useful for making of the nickel-carbon nanofiber materials can operate up to a maximum working pressure of about 2000 pounds per square inch and a maximum temperature of about 800° C. Various reaction parameters such as heating rate, temperature, duration, reactant concentration, stoichiometry, pressure, and atmosphere (either oxidizing, reducing or inert) can be controlled to alter the physical and chemical nature of carbon-containing products formed in such reactors. Preferably, the reactor operates under conditions ranging from a minimum working pressure of about 100 pounds per square inch and a minimum temperature of about 100° C., to a maximum working pressure in the range of about 800 to about 2000 pounds per square inch and a maximum temperature in the range of about 300 to about 800° C.

Liquid Lubricant Test-Sample Preparation.

NiCNF materials as described herein were mixed with and dispersed in a selected base-lubricant (e.g., a poly(alpha olefin) or 5w30 oil). Typically, the nanofiber material is stirred in the liquid medium continuously at about 60° C. for approximately 1 hour prior to being evaluated.

Tribological Test Set-Up.

The tribological performance of the test lubricants was evaluated using the facilities at the Tribology Section at Argonne National Laboratory. Tests were conducted at extremely high contact pressures and at elevated temperatures that are representative of the typical operating conditions of real automotive/industrial components. A Ball-on-Disc Tribo-tester was utilized for the tribological evaluations. The ball was loaded on top of the plate in order to create a contact pressure of about 0.61 to about 1.05 GPa. The tests were conducted at various temperatures from about 20 to about 100° C. and in open air. The plate is rotatable at variable speeds and is pressed and slid against the ball sample during operation.

Post-Test Analysis.

Once, the tribological tests were completed, the test samples (ball and plate) were retrieved and cleaned using solvents. After cleaning, the worn areas examined using an optical microscope or subjected to other microscopic and/or spectroscopic techniques.

Example 1 Preparation of a Nickel-Carbon Nanofiber Material

About 1 gram (g) of nickel(II) acetate was heated at about 700° C. in an inert nitrogen atmosphere for about 1 hour in a closed reactor under autogenic (self-generating) pressure to afford about 0.05 g of a carbon nanofiber material. Individual fibers of the material had lengths in the range of about 100 nm to about 1 μm (average length of about 500 nm) and an average fiber diameter of about 40 nm, with nickel nanoparticles having diameters of about 10 to about 25 nm (average diameter of about 15 nm) encapsulated at the tip of the nanofiber.

Scanning electron microscope images of a nanofiber from the obtained product are shown in FIG. 1, Panels (a) and (b). Panel (a) of FIG. 1 provides an electron micrograph of solid nickel-containing carbon nanofiber 10 having a carbon nanofiber shaft 12 with a diameter about 30 to 40 nm, capped with a nickel nanoparticle 14 having a diameter of about 18 nm. Panel (b) of FIG. 1 shows a high resolution image of the a portion of the nickel nanoparticle 14 with several layers of graphic carbon 16 encapsulating nanoparticle 14 at a thickness of about 7 to 8 nm.

Example 2 Tribological Testing of PAO-Based Lubricants

Tribological testing was conducted using the Ball-on-Disc Tribo-tester as described herein using a liquid lubricant medium of a PAO: Comparative Lubricant A (PAO alone), and Lubricant B (0.1 wt % NiCNF of Example 1 dispersed in PAO). The PAO had a viscosity of about 4 centiStokes (cSt) was tested under the following conditions: maximum pressure of about 0.61 GPa, a speed of 3 mm/sec, and a Hertz contact diameter of about 80 mm, at temperatures of 20, 80 and 100° C. The base lubricant, Comparative Lubricant A, exhibited a friction coefficient of about 0.13 to about 0.15, while Lubricant B, comprising the NiCNF material exhibited a friction coefficient of about 0.10, at the tested temperatures.

FIG. 2 illustrates micrographic images of wear scars observed in evaluation of Lubricant B (NiCNF-containing lubricant) at 20° C. in Panel (a), and at 100° C. in Panel (c). Panel (b) shows a magnification of the wear scar of Panel (a); while Panel (d) shows a magnification of the wear scar of Panel (c). The nature of the wear scar was analyzed by energy-dispersive X-ray spectroscopy. The results are shown in Table 1, which lists the elemental composition of the wear scar material as well as the ZAF correction matrix factors (Z=atomic number effect, A=X-ray absorption effect; and F=X-ray fluorescence effect).

TABLE 1 Elem Line Wt % Atom % K-Ratio Z A F C Kα 11.84 39.09 0.0358 1.1980 0.2525 1.0002 As Lα 0.77 0.41 0.0036 0.8775 0.5375 1.0000 Fe Kα 41.83 29.71 0.4164 0.9597 0.9999 1.0373 Ni Kα 45.56 30.78 0.4374 0.9680 0.9920 1.0000 Total 100.00 100.00

As is evident from the data in Table 1, the NiCNF material surprisingly deposited a nickel-rich boundary film in the wear scars, resulting in an effectively negative wear and at least partial repair of the wear scar. By way of comparison, Lubricant A (PAO alone) exhibited a more significant wear scar at each temperature tested.

Example 3 Tribological Testing of 5w30-Grade Lubricants

Tribological testing was conducted using the Ball-on-Disc Tribo-tester as described herein using a fully formulated lubricant of 5w30: i.e., Comparative Lubricant C (5w30 alone), and Lubricant D (0.1 wt % NiCNF of Example 1 dispersed in 5w30). The lubricants were tested under the following extreme test conditions: maximum pressure of about 1.05 GPa, a speed of 30 mm/sec, and a Hertz contact diameter of about 140 mm, for one hour at a temperatures of 20° C. The formulated lubricant, Comparative Lubricant C, exhibited a friction coefficient of about 0.16, while Lubricant D, comprising the NiCNF material exhibited a friction coefficient of about 0.10, at the tested temperature.

FIG. 3 illustrates micrographic images of wear scars observed in evaluation of Lubricant D (NiCNF-containing lubricant) in Panel (a), and Comparative Lubricant C (5w30 alone) Panel (b). As in Example 2, the NiCNF-containing lubricant exhibited a significantly less severe wear scar due to the surprising deposit of a nickel-containing boundary film in the scar.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The terms “consisting of” and “consists of” are to be construed as closed terms, which limit any compositions or methods to the specified components or steps, respectively, that are listed in a given claim or portion of the specification. In addition, and because of its open nature, the term “comprising” broadly encompasses compositions and methods that “consist essentially of” or “consist of” specified components or steps, in addition to compositions and methods that include other components or steps beyond those listed in the given claim or portion of the specification. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All numerical values obtained by measurement (e.g., weight, concentration, physical dimensions, removal rates, flow rates, and the like) are not to be construed as absolutely precise numbers, and should be considered to encompass values within the known limits of the measurement techniques commonly used in the art, regardless of whether or not the term “about” is explicitly stated. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate certain aspects of the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A nickel-containing nanofiber (NiCNF) material comprising a plurality of elongate nickel-containing carbon nanofibers, wherein individual nanofibers of the NiCNF material include a nickel nanoparticle encapsulated within multiple layers of graphitic carbon at one end of a carbon nanofiber shaft.
 2. The NiCNF material of claim 1 wherein the individual nanofibers have a fiber diameter in the range of about of about 30 to about 100 nanometers (nm).
 3. The NiCNF material of claim 1 wherein the individual nanofibers have a length in the range of about 100 nm to about 2 micrometers (μm).
 4. The NiCNF material of claim 1 wherein the nickel nanoparticle has a diameter in the range of about 10 to about 30 nm.
 5. The NiCNF material of claim 1 wherein the graphitic carbon encapsulating the nickel nanoparticle has a thickness in the range of about 4 to about 10 nm.
 6. The NiCNF material of claim 1 wherein the NiCNF material contains not more than about 60 wt % of the carbon that is not part of the nickel-containing carbon nanofibers.
 7. The NiCNF material of claim 1 wherein at least about 50% of the nickel in the NiCNF material is encapsulated at an end of a carbon nanofiber shaft.
 8. The NiCNF material of claim 1 wherein the individual nanofibers have a fiber diameter in the range of about of about 30 to about 100 nm, and a length in the range of about 100 nm to about 2 μm; the nickel nanoparticle has a diameter in the range of about 10 to about 30 nm; and the graphitic carbon encapsulating the nickel nanoparticle has a thickness in the range of about 4 to about 10 nm.
 9. A lubricant composition comprising the NiCNF material of claim 1 dispersed in a liquid medium.
 10. The lubricant composition of claim 9 wherein the individual nanofibers have a fiber diameter in the range of about of about 30 to about 100 nm, and a length in the range of about 100 nm to about 2 μm; the nickel nanoparticle has a diameter in the range of about 10 to about 30 nm; and the graphitic carbon encapsulating the nickel nanoparticle has a thickness in the range of about 4 to about 10 nm.
 11. The lubricant composition of claim 9 further comprising a surfactant.
 12. The lubricant composition of claim 11 wherein the surfactant comprises a non-ionic surfactant.
 13. The lubricant composition of claim 11 wherein the surfactant comprises sorbitan trioleate.
 14. The lubricant composition of claim 11 wherein the surfactant is present in the composition at a concentration in the range of about 1000 to about 20000 parts-per-million (ppm).
 15. The lubricant composition of claim 9 wherein the liquid carrier comprises a liquid hydrocarbon.
 16. The lubricant composition of claim 15 wherein the liquid hydrocarbon comprises a poly alpha olefin.
 17. The lubricant composition of claim 9 wherein the NiCNF material is present in the composition at a concentration in the range of about 0.01 to about 15 wt %.
 18. The lubricant composition of claim 9 wherein the liquid carrier comprises a poly alpha olefin and the NiCNF material is present in the composition at a concentration in the range of about 0.5 to about 1 wt %.
 19. The lubricant composition of claim 18 further comprising a non-ionic surfactant.
 20. A method of preparing a NiCNF material of claim 1 comprising heating nickel(II) acetate neat in a sealed pressure reactor at a temperature of about 650 to about 800° C. at an autogenically produced pressure of about 300 to about 600 psi under an inert atmosphere. 